First, some math I did trying to work figure out our confusion. I'm not sure if it helps or is still worth posting so feel free to skip this part.

[math]

Let X be the maximum number of times you can play Pirate Ship in a turn. On your Nth turn, you could play a single Pirate Ship, choosing +Y coins. The maximum value for Y is X*N - 1; every time you played pirate ship, you took a coin token except for the time you chose to play it for +Y coins, which would be the last possible time you could play Pirate Ship on your Nth turn.

Let B be the maximum number of buys you can make on a turn (in a deck where you can reliably play all 10 Merchant Guilds). You've spent N-1 turns stockpiling Coin tokens from Merchant Guild, and you're going to use all of them on your Nth turn. The maximum number of coins you can choose to spend on your Nth turn is B*10*(N - 1); on every turn before, you've had 10 Merchant Guilds in play and used all your buys (thanks to Trader and an empty Silver pile).

pirate ship max: X*N -1

merchant guild max: (N-1)*10*B

These maxes are of course still too high--when you're on the way to setting up perfect turns with max buys or max plays of pirate ship, you'll be taking far fewer coin tokens--but they're much easier upper bounds to calculate than the actual maxes.

[/math]

I think I figured out the confusion/miscommunication...

I didn't say "infinite" originally because that would require infinite turns or some crazy loop allowing you to Pirate Ship infinite times in one turn. "Unbounded" probably wasn't the right term either, since it is actually bounded by the number of turns taken before choosing to spend the coins.

It looks like maybe you were thinking of income from a single play of a card, while I was thinking of income from a card, allowing for the influence of previous turns (which I guess you pointed out when you brought up Candlestick Maker).

It seemed to me like you were pointing out that the growth of Merchant Guild's potential income was limited (which clearly does apply to Pirate Ship), not that you were saying a single Merchant Guild play could only get you so far (i.e., B is the important number, rather than (N-1)*10*B).

The post that started this discussion didn't strictly define "highest potential income actions", and I think we just took different angles at:

Am I right in thinking that vault, storeroom, and secret chamber are the highest potential income actions in Dominion?

Sorry for misunderstanding, and hopefully this long post didn't make it worse